1. Field of Invention
The invention relates generally to non-ideal light sources, and more specifically to methods and apparatuses for utilizing non-ideal light sources in scanned beam devices, such as displays and imaging devices.
2. Art Background
Light sources are used in devices used for image capture and in devices used to display an image to a user. Two parameters of a light source, used for such devices, are amplitude stability of the optical output as a function of time and the general stability of the dominant wavelength of the spectral distribution of optical energy. Fluctuations of either one or both of these parameters can cause a number of undesirable effects in either the image captured or the image displayed.
Several variations (non-idealities) of light source amplitude (optical intensity) are shown in FIG. 1, where amplitude 102 is plotted as a function of time 104. The preferred optical output is stable over time as indicated at 106. Many real light sources exhibit one or more of the non-idealities illustrated in FIG. 1 and FIG. 2.
Some examples of light sources used in the devices described above are a light emitting diode (LED), an edge emitting light emitting diode (EELED), a laser diode (LD), a diode pumped solid state (DPSS) laser, etc. One non-ideality these devices can exhibit is shot noise as shown at 114. Another non-ideality is a temporally periodic amplitude fluctuation 112. The amplitude can also decay with time as shown at 108 or increase with time as shown at 110. The amplitude non-idealities are not meant to be plotted on a common time scale 104 in FIG. 1, but are merely over plotted on the same time scale 104 for ease of discussion.
An optical output, spectral power density, (SPD) 202 of a light source is plotted a function of wavelength 204 in FIG. 2. Light sources such as InGaN-based blue and green edge emitting light emitting diodes (EELEDs), LDs, as well as other light sources, exhibit a drive level dependent output spectra. Such a drive level dependent output spectra causes a first spectrum with a dominant wavelength 206 to shift to a second spectrum with a dominant wavelength 208. This may cause various problems when such a light source is used in devices used to capture or display images.
For example, FIG. 3 shows a shifting color gamut, generally at 300, due to the effects described in conjunction with FIG. 2. With reference to FIG. 3, red, green, and blue (RGB) light source outputs 308, 310, and 312 are plotted in a Commission Internationale de l'Eclairage (CIE) color space with yc corresponding to 302 and xc corresponding to 304, and a region interior to curve 306 indicates the envelope of colors perceivable to the human eye. The triangle formed by connecting 308, 310, and 312 has a white point WP at 314. These RGB values can correspond to a particular light source drive level and 310 (FIG. 3) can correspond with dominant wavelength 206 (FIG. 2). At another drive level, the spectral output of the green source shifts to 210 (FIG. 2) causing a shift in the green dominant wavelength G plotted at 310 to a perturbed spectrum G′ plotted at 316. In this example, the peak wavelength of the green source not only shifted, but the spectral purity of the source also shifted, as shown by the shorter, broader spectrum 210. Similarly, the blue light source can experience a shift in dominant wavelength from B to B′ with drive level, resulting in a shift of CIE chromaticity from 312 to 318. Ignoring the relative values of green and blue spectra, the change in spectral output corresponds, for example, to a shift from curve 206 to 208 (FIG. 2). A new color gamut is formed by the triangle represented by 308, 316, and 318 (in this example the red light source is assumed to be unaffected by drive level changes). The white point WP′ within the new color gamut corresponds to point 320, which represents a shift from the white point WP at 314. Such drive level dependant color gamut fluctuations are often undesirable.
It is generally desirable to use inexpensive light sources in order to reduce the cost of a display or an image capture device. However, inexpensive light sources may tend to exhibit the above described non-idealities to an unacceptable level. For example, an inexpensive DPSS laser can exhibit amplitude fluctuations of 30 percent. The amplitude noise on some DPSS lasers occurs in the 1-100 kilohertz band, which can coincide with the periodicity or other features of some image data, to make the image artifacts even more pronounced during either image capture or image display.
Polarization can also fluctuate and, owing to polarization sensitivities in some systems, can produce similar amplitude variations. Typically, such amplitude variations result from polarization dependent differences in system gain. Additionally, DPSS and similar sources may exhibit mode coupling, where the beating of a plurality of modes can produce amplitude noise.
While the tolerance of the eye to amplitude variations is image dependent and spatial frequency dependent, the human eye generally can detect two (2) to three (3) percent amplitude variations in small regions. Thus, amplitude variations in such light sources may produce perceivable image artifacts.